Liquid crystal-based widely tunable integrated optic wavelength filters

We report widely tunable wavelength filters using liquid crystal (LC)-based long-period waveguide gratings (LPWGs). The high birefringence and electrically tunable refractive index of the LC have been used to achieve a tuning range covering most of the S, C, and L bands of an optical communication system. E7 LC has been used as a cover layer in an LPWG rib waveguide structure. The LPWG structure couples light from the fundamental mode to the slab modes of the waveguide. This results in transmission dips at resonance wavelengths. The mode constants of the coupling modes can be changed by controlling the refractive index of the LC and hence the resonance wavelengths by using the externally applied electric field. We have carried out numerical simulations to analyze the performance of E7 LC-based LPWGs on various substrates such as ion-exchanged BK7 glass and proton-exchanged LiNbO3 and silicon-on-insulator and silicon nitride substrates. The potential applications of the device are electrically tunable wavelength filters for optical communication systems.